In recent years, the popularity of and the development in optical devices have driven the need for optical glasses having a wide range of characteristics. Especially, a fluorophosphate optical glass is in great demand for its properties, such as low dispersibility, abnormal dispersibility, and high light transmittance in a wide range of the visible region. With its low dispersibility and abnormal dispersibility, the fluorophosphate optical glass is effective for the correction of chromatic aberration. With its high light transmittance, the fluorophosphate optical glass is also effective not only as a material of an optical element in an imaging optical system but as a material of an optical element for guiding short wavelength light such as near ultraviolet light. Furthermore, the fluorophosphate optical glass that is added with copper ions may be imparted with a filter function for cutting near red light, serving as a material of a color correction filter in a semiconductor imaging device.
Conventionally known fluorophosphate optical glasses include (i) the one having a refractive index (nd) of 1.58183 and an Abbe number (νd) of 67.6 (refer to Patent Literature 1, for example), (ii) the one that contains, as cationic components, P+ in an amount of 39.0%, Al3+ in an amount of 12.0%, Mg2+ in an amount of 12.0%, and Ba2+ in an amount of 29.3%, and as anionic components, O2− in an amount of 75.5% and F− in an amount of 24.5%, and that has a refractive index (nd) of 1.5909 and an Abbe number (νd) of 67.9 (refer to Patent Literature 2, for example), and (iii) the one that contains, as cationic components, P+ in an amount of 39.0%, Al3+ in an amount of 12.0%, and Y3+ in an amount of 2.4%, and as anionic components, O2− in an amount of 73.3% and F− in an amount of 26.7%, and that has a refractive index (nd) of 1.5881 and an Abbe number (νd) of 68.0 (refer to Patent Literature 3, for example).
However, the optical glasses described in Patent Literatures 1 to 3 have not been satisfactory yet in terms of specific gravity, abrasion degree, and devitrification-proof stability.